The invention relates generally to processes for generating ozone. More specifically, this invention relates to a method for optimizing the amount of ozone that can be generated with a corona discharge ozone generator.
Ozone is an unstable triatomic allotrope of oxygen. It is produced in an energized environment wherein molecular oxygen dissociates into monatomic oxygen, which subsequently collides and recombines with an oxygen molecule to form a highly reactive ozone molecule.
Although ozone is primarily employed in disinfection, it can perform other functions such as color reduction, odor and taste removal, algae control, oxidation of inorganic and organic compounds in water and waste-water treatment practices, waste gas treatment and bleaching of paper pulp.
The most prominent features of ozone as a biocide lie in its speed and selectivity in oxidation. Biocidal effects are believed to be primarily achieved through oxidation. Consistent with this belief, the ability of any chemical to reduce microbial viability is in direct proportion to its oxidation potential. Ozone is the fourth most powerful oxidizing agent known; only fluorine, fluorine dioxide, and monatomic oxygen are thought to be more reactive. Ozone possesses an oxidation potential of 2.07 millivolts relative to an oxidation potential of 1.36 millivolts for chlorine gas. It is important to note that an increased oxidation potential is indicative of an accelerated bacterial kill. The rate of disinfection has been demonstrated to be more than 3,000 times faster than chlorine; thus, contact time is a lesser consideration in the application of ozone as a microbicide.
Disinfection with the use of ozone may proceed by oxidation directly and by intermediate hydroperoxy compounds that can interact with cytosolic components. Organic ozone chemistry would predict that oxidized organic compounds containing carbon-carbon double bonds give rise to hydroperoxyalcohols. Evidence exists that organic peroxides exert a stronger bactericidal action than hydrogen peroxide due to a greater tendency to decompose. No evidence is believed to exist in the literature of any microorganism that is resistant to the effects of ozone exposure. The application of ozone is preferable due to its compatibility with biota. There are no residual or harmful reaction products downstream, particularly in the range of 0-20 ppm. The presence of peroxidic compounds could be perceived to be harmful to the biota, but toxicity studies indicate the contrary to be true. Studies have shown that, chemically, these compounds are highly unstable and rapidly decompose. It has also been shown that these compounds can be removed by other oxidizing molecules.
In addition to demonstrating powerful capabilities in the destruction or inactivation of bacteria, fungi and protozoa, ozone has been shown to be virucidal. The efficacy of ozone (a 99% reduction was reported for all of the following values given) has been reported to range from 2.2 mg/l for Escherichia coli in 19 minutes from raw waste water; 0.02 mg/l for Candida tropicalis in 0.30 minutes from ozone-demand free water; 1.2 mg/l for Naegleria gruberi in 1.1 minutes from ozone-demand free phosphate buffer solution and 0.2 mg/l for Poliovirus type I in 9 minutes from activated sludge effluent. With regard to bacterial spores (specifically, Bacillus subtilis v.globigii), ozone has been shown to achieve a four-log reduction within the range of 1.5 minutes to 2 minutes when water is purged with 3% ozone, by weight. Using a non-toxic concentration of 4 xcexcg ozone per ml of serum, ozone can achieve a six-log reduction in the infectious titer of human immunodeficiency virus (xe2x80x9cHIVxe2x80x9d).
Ozone can be created by an ozone generator, which subjects air to an electric discharge that is referred to in the art as a corona When air is forced through the corona, oxygen in the air is transformed from O2, the molecule containing two oxygen atoms, which is normally found in the air, into single oxygen-free radicals, Some of these free radicals combine to form ozone. An exemplary corona discharge ozone generator is disclosed in U.S. Pa. No. 5,145,350 to Dawson et al., Sep. 8, 1992 (hereinafter xe2x80x9cthe ""350 Patentxe2x80x9d), the disclosure of which is hereby incorporated by this reference in its entirety.
The ozone generator of the ""350 Patent includes a drive circuit and a xe2x80x9ccorona cellxe2x80x9d, which includes a hollow insulator, a primary helical electrode positioned around the outside of the insulator, and a secondary electrode positioned within the insulator. At least one of the primary and secondary electrodes of the corona cell is spaced apart from the insulator. The drive circuit of the ozone generator includes a waveform generator, which generates a direct current (xe2x80x9cDCxe2x80x9d) voltage. A DC switching device associated with the waveform generator rapidly applies a DC voltage to the primary winding of a transformer that is connected to the corona cell and withdraws the DC voltage from the primary winding.
As the DC voltage is suddenly applied to the primary winding of the transformer, magnetic fields are developed which excite a secondary winding of the transformer and a circuit of the corona cell. Similarly, when the voltage is withdrawn from the primary winding, the magnetic fields collapse, which also excites the secondary winding of the transformer and the corona cell. Unless these oscillations in the voltage of the secondary winding of the transformer and corona cell are sustained or dampened, they will eventually decay. The DC voltage is repeatedly applied to and withdrawn from the primary winding of the transformer to excite the secondary winding of the transformer at a frequency that results in a sufficient potential difference between the primary and secondary electrodes of the corona cell that one or both of the electrodes discharge electricity into the space between at least one of the electrodes and the insulator of the corona cell. As oxygen molecules pass through the space while electricity is being discharged therein, the oxygen molecules (O2) are broken up into oxygen free radicals, some of which combine with oxygen molecules (O2) to form ozone (O3).
The secondary winding of the transformer and the corona cell form a xe2x80x9ctank circuitxe2x80x9d, which has a resonant frequency (f0). The resonant frequency of the tank circuit depends on the inductance of the secondary winding of the transformer and the capacitance of the corona cell, which is dictated, at least in part, by the material and thickness of the insulator. Typically, the frequency with which the DC voltage is applied to the primary winding of the transformer by the waveform generator, commonly referred to as the driving frequency, is approximately equal to the resonant frequency of the tank circuit.
It would be an improvement in the art to produce ozone in increased concentrations while using known corona discharge ozone generators.
The invention includes a method for operating corona discharge ozone generators in a manner that optimizes the concentration of ozone that can be generated thereby. The method of the invention includes determining the resonant frequency of the tank circuit of the corona discharge ozone generator and applying a direct current to a primary winding of a transformer of the ozone generator at a driving frequency equal to about half the tank circuit""s resonant frequency.
In another aspect of the method of the present invention, the concentration of ozone created by a corona discharge ozone generator is maximized by selecting a corona cell with a capacitance that produces a resonant frequency equal to about two times a driving frequency applied to a primary winding of a transformer of the ozone generator.
The methods of the present invention may be used to provide an increased concentration of ozone in a variety of applications, including the treatment of liquids and sanitization of foods.